It is known that electromagnetic communication systems employ broad bandwidth techniques, such as the so-called frequency-agile or frequency-hopping systems in which both the transmitter and receiver rapidly and frequently change communication frequencies within a broad frequency spectrum in a manner known to both units. When operating with such systems, antennas having multiple matching and/or tuning circuits that must be switched, whether manually or electronically, with the instantaneous frequency used for communications, are simply inadequate. Instead, it is imperative to have a single antenna reasonably matched and tuned to all frequencies throughout the broad frequency spectrum of interest. One attempt to provide such an antenna is disclosed in U.S. Pat. No. 4,958,164, which is incorporated herein by reference, and which is owned by the Assignee of the present invention. Although adequate in its stated purpose, the above-identified invention does not adequately perform in a higher frequency range.
Initial attempts at providing an antenna with characteristics of instantaneous bandwidth in a higher frequency and having a VSWR no greater than 2:1 were not found to be acceptable. As is known in the art, the bandwidth of an antenna is related to the "Q" of the antenna. The Q or quality factor/selectivity of an antenna can be defined as power stored in the electrical fields surrounding the antenna divided by the power radiated into space, or more simply, power stored divided by power radiated. It is well known that the lower the Q of the antenna, the larger the antenna bandwidth. The major factors affecting antenna Q are: the length/diameter ratio of the radiator; the antenna loss as related to the loss associated with conductor heat loss; placing a load such as a coil or capacitor in series at some point in the radiator; and, matching networks that may be required at the antenna base in order to match the antenna to the required 50 ohms transmission line.
A proposed quarter wave antenna was found to be unacceptable as it exhibited approximately 36 ohms radiation resistance and no reactance at the quarter wave frequency. Although no matching elements were required at the base of the radiating element, at frequencies lower than the quarter wave frequency the radiation resistance decreased and the capacitance reactance increased. At frequencies higher than the quarter wave frequency, the radiation resistance increased and reactive component acted as a series inductance with increasing reactance with increasing frequency. Based upon such an initial construction, it was found that the quarter wave antenna would be marginal at best. In particular, it was found that the VSWR requirement could only be met between the frequencies of 150 to 165 MHz. Outside of this range, the VSWR exceeded the necessary requirements.
A 5/8 wave antenna nominally exhibits approximately 50 ohms radiation resistance and a high capacity reactance at its operating frequency. In these types of construction, it is customary to insert at the base of the antenna a "matching" inductance to cancel the capacitor reactance so that the antenna system impedance is 50 ohms resistive. Although the 5/8 wave antenna is well suited to mobile installations, it was found to not meet the desired low "Q" and broad band bandwidth requirements. Such an antenna construction was found to have only an acceptable frequency range of about 148 MHz to about 158 MHz within the specified VSWR range. Accordingly, such a construction was found to be unacceptable.
One construction that was found to be promising was a capacitance loaded antenna which had a base radiator section of approximately 128.5 electrical degrees of one wavelength and a tip radiator section of approximately 64.25 electrical degrees of one wavelength. This construction includes a series insertion of a capacitance of approximately 2 pf between the base and tip radiators. This provides an antenna with the desired VSWR characteristic between 140 and 170 MHz. Unfortunately, the desired VSWR characteristics were not obtained at the extremes of the frequency bandwidth desired.